When I awoke this morning my FreeBSD box was frozen -- completely unresponsive
at the console -- and displayed these messages on the console:
ad0: READ command timeout tag=0 serv=0 -resetting
ata0: resetting devices
ad0: removed from configuration
done
Aug 30 03:09:25 abc sm-mta 88427 xyz SYSERR(root): collect: Cannot write:
./xyz (fsync uid=o, gid=25): Device not configured
The box has been in continuous operation for years, and I've never seen this
behavior before.
I rebooted the machine this morning and, thank God, it came back to life.
Is this an early warning that the hard drive is failing and should be replaced
this week?

Very probably!

Is there anything else I should explore or do at this time?

If not the drive, check drive cables, the power supply and
motherboard/CPU overheating.